Peripheral blood cytopenias in patients with systemic infection are well-established predictors of fatal outcome despite antimicrobial therapy. This project proposal seeks to explain the molecular mechanism of sepsis- induced cytopenias. We have identified a central role for Caspase-1-dependent death, known as pyroptosis, of hematopoietic stem and progenitor cells in infection-induced cytopenias. We have generated two NLRP1 mutant mouse models, the first an ENU-induced gain-of-function mutation in NLRP1a (NLRP1aQ593P), and the second a deficiency in all three isoforms of NLRP1 (NLRP1a, NLRP1b and NLRP1c). The NLRP1aQ593P mutation occurs in the flexible linker region between the nucleotide-binding (NACHT, NB) domain and leucine- rich repeats (LRR), where activating mutations that reduce the threshold for activation of plant NB-LRR innate defense proteins are found. Nlrp1aQ593P/Q593P mice develop lethal meningoencephalitis, pneumonitis and myocarditis. This disease is dependent on IL-1 and Caspase-1 but not ASC or Caspase-11 and is unexpectedly, negatively regulated by IL-18. When Nlrp1aQ593P/Q593P mice are deficient in the IL-1 receptor, the multi-organ inflammatory disease does not develop but the mice are leukopenic and they display significant deficiencies in hematopoietic stem and progenitor cells at the steady state (Table 1). In response to hematopoietic stress induced by lymphocytic choriomeningitis virus (LCMV) infection or chemotherapy, Nlrp1aQ593P/Q593P mice fail to repopulate myeloid, lymphoid and erythroid cells. Conversely, NLRP1-deficient mice show enhanced recovery from LCMV infection and chemotherapy. Based on our preliminary data, we propose (1) that systemic activation of NLRP1 in hematopoietic progenitor cells during infection can induce cytopenia resulting in immunosuppression. We also propose that (2) one of the physiological roles of NLRP1 is to induce Caspase-1-dependent death of infected hematopoietic stem and progenitor cells to prevent dissemination of infection to progeny cells. This research seeks to: A. examine the role of biochemical regulators of NLRP1 including Bcl-2 and SREBP-1a to identify new avenues to modulate NLRP1 in vivo B. define specific hematopoietic stem and progenitor cell populations affected by NLRP1 using competitive bone marrow transplant, serial bone marrow transplant, and ex vivo clonal culture and viability assays. C. characterise the role of NLRP1 in hematopoietic stem and progenitor cells during hematopoietic stress induced by infection and chemotherapy, to understand beneficial and pathological roles for NLRP1 activation